Brooks.] 194 [November 17, 



is inclosed within a capsule of epithelial cells, and is suspended in 

 the sinus system of the " zooid " on the neural side, between the 

 stomach and the atrial orifice, by means of a gubernaculum, by which 

 it is attached to the wall of the branchial sac. (See Figure I.) 



The ovum shows no trace of a vitelline membrane; the yolk is 

 composed of transparent protoplasm without granules, and the germ- 

 inal vesicle contains no dot, but seems to be homogeneous. 



Impregnation takes place through the action of the spermatic fila- 

 ments which are discharged into the water by the " zooids " of other 

 fullgrown chains, are drawn into the branchial sacs of the immature 

 " zooids " which contain the eggs, and penetrate into the interior of 

 the gubernaculum. 



Upon impregnation the germinative vesicle disappears; the guber- 

 naculum becomes irregularly swollen and shortened, thus drawing 

 the egg down into the brood-sac, which is formed by an involution of 

 the branchical sac of the nurse (Figure II). The egg, nourished by 

 the blood which bathes it, rapidly increases in size, and undergoes a 

 process of total segmentation, as the result of which two portions are 

 formed; a finely segmented "germ yolk," and a less completely 

 segmented u food yolk." (Figure V.) 



The latter becomes enveloped by the former through a process of 

 invagination, forming. a true " gastrula" or " invaginate planula," the 

 opening of which, the " orifice of Rusconi," persists and forms the 

 orifice of the placenta. (Figures VI, VII, VIII, /.) 



The embryo, still growing rapidly, becomes divided into two por- 

 tions by a constriction (Figure VII) ; the portion nearest the point of 

 attachment to the brood sac forms the embryo proper, and the re- 

 maining portion that part of the placenta which is to be in communi- 

 cation with the sinus system of the foetus. (Figure VII.) 



Within this portion there is a cup-shaped cavity, part of the origi- 

 nal "cavity of Rusconi," which is in direct communication with the 

 sinus system of the nurse, and thus forms the second or inner cham- 

 ber of the placenta. This soon becomes divided up into a great 

 number of irregular intercommunicating lacunas, which are produced 

 by the growth of a structure resembling a stump with its roots, and 

 which seems to be formed directly from the blood of the nurse, by 

 the aggregation and fusion of the blood corpuscles. 



The subsequent development of the foetus, which is the young of 

 the solitary salpa, is substantially as it has been described by Sars, 



